1. Field of the Invention
The present invention relates to the fabrication of low-cost photovoltaic cells primarily for terrestrial applications by plasma spraying techniques. More particularly, it is concerned with a process for plasma spraying polycrystalline silicon films, the subsequent formation of p-n junctions in sprayed films by the use of additional plasma spraying, diffusion, or ion implantation, and the utilization of such p-n junctions to form solar cells which are useful for producing electricity directly from sunlight via the photovoltaic effect. Electrical conductors and antireflective coatings can also be formed on solar cells by plasma spraying or by other means.
2. Description of the Prior Art
Because of the rapid depletion of our natural energy sources, such as domestic oil and natural gas, the pollution problems associated with the use of coal and the slow development of nuclear energy, other energy sources have assumed a new importance. Solar energy is abundantly available in this country and can be utilized without ecological problems. Electricity can be generated from solar energy by sequential conversion (i.e., sunlight to heat, to steam, to mechanical power, to electricity) or by direct conversion via the photovoltaic process. In the photovoltaic effect, an electromotive force is generated when light is allowed to fall upon a rectifying contact or junction such as that contained in a solar cell.
Conventional solar cells are fabricated from single crystal silicon and used mostly in space applications. Single crystal silicon wafers are produced by preparing high quality ingots of doped silicon followed by precise cutting, lapping, and chemical polishing. Another dopant of opposite polarity to that used in the ingot is then diffused into the surface of the wafer to yield a p-n junction. The diffused layer is removed from the back surface and ohmic contacts are applied to that surface. Grid contacts are applied to the diffused surface followed by an antireflective or protective coating. Cells are then mounted on a supporting structure and electrical leads are attached. Conversion efficiencies of 10 to 14% have been observed in solar cells, prepared from single crystal silicon.
A very expensive portion of the process described above for preparing solar cells from single crystal silicon is the preparation of high quality single crystal silicon and its conversion into wafers by precise slicing and polishing procedures. If such single crystal silicon were used in large area solar cells for terrestrial applications, costs would be prohibitive and result in very limited utilization. Polycrystalline silicon with satisfactory properties can be produced and used to form solar cells with acceptable performance characteristics on a low-cost mass-production basis and process economics make costs acceptable for large scale utilization.
In the prior art, polycrystalline silicon films have been produced by chemical vapor deposition and by evaporation. Both processes are slow, requiring much time to produce films of useful thickness, and both processes are conducted in a vacuum environment. The plasma spraying process described herein is a fast deposition process, conducted at essentially atmospheric pressure, capable of the large scale production of low-cost polycrystalline silicon photovoltaic cells in large-area form on an automated, continuous basis. The process is capable of depositing not only polycrystalline silicon but also electrical contacts, antireflective or protective coatings, an other semiconductor materials.
The plasma generators used in this invention are well-known in the art and commercially available. They are electrical devices used for continuously generating a high temperature gas which is partially or completely ionized (called a "plasma"). The high-temperature gas is used for the melting of injected particles and the spraying and deposition of molten particles onto a substrate to solidify and form a polycrystalline film. By injecting doped silicon particles into the hot gas, doped polycrystalline silicon films can be produced. Films can be deposited on an electrical conducting substrate such as a metal or graphite which can then serve as ohmic contact in the solar cell. Metallized nonconductors can also be used as substrates. Free-standing silicon films can be produced by depositing onto removable substrates.